This invention relates to methods and apparatus for use in determining the spatial location of the points on an object surface and, in particular, to an improved method and apparatus of this type.
A variety of different types of systems have been used to determine the spatial location of the points on an object surface. One type of system is disclosed in U.S. Pat. Nos. 4,199,253 and 4,854,698, assigned to the assignee hereof, and the teachings of which are incorporated herein by reference.
In the system of the '253 patent, optical energy is first projected at the object surface. The object surface then reflects certain of the projected optical energy and the reflected optical energy is collected by a receiver. At the receiver, the time delay of receipt of each of a number of individual parts of the received reflected optical energy is determined. This time delay is indicative of the outward distance (i.e., the so-called "range") of the surface point corresponding to the particular received energy part. The time delay information along with other two-dimensional information contained in the received optical energy and with the position of the lens node of the receiver are then used to locate surface points in three dimensions using standard triangulation processing.
In the '253 patent system, time delay information is developed by using different preselected time dependent gating functions to gate a shutter means in the receiver. These time dependent gating functions together encode the object surface points into corresponding time delay intervals. As a result, the presence or absence of an individual optical energy part in the reflected optical energy received during the different gating functions provides a code from which the time delay interval for the surface point corresponding to the optical energy part can be determined.
The '253 patent further contemplates the use of multiple receiver channels and a single projected pulse as one mechanism for subjecting received optical energy to the different gating functions. Also contemplated is a second mechanism in which a succession of optical pulses are transmitted over a single channel and the different gating functions are applied to the resultant succession of optical signals received over the single channel.
The ability of the digital type of system disclosed in the '253 patent to provide fine spatial resolution of the object surface points requires the use of a large number of channels or a large succession of pulses and complex circuitry for generating the gating functions. This has prompted continued efforts to design systems for surface point determination which are less complex.
One such system is disclosed in U.S. Pat. No. 4,935,616, the teachings of which are also incorporated herein by reference. The system of the '616 patent is an analog type system in which a continuously and sinsusoidally modulated optical laser beam is projected at an object surface. The optical energy reflected from the surface is then received in a receiver designed to determine the phase shift of the received energy parts relative to a reference signal. The measured phase shift then serves as an indication of the range of the energy part and, therefore, its corresponding surface point.
In the receiver of the '616 patent, the received optical energy first undergoes conversion to an electron beam. During this conversion the energy is modulated by a sinsusoidal signal at the same frequency as the original modulation frequency of the projected signal. The modulated electrical signal is then converted back to an optical signal and the resultant optical signal conveyed to an array detector. Each cell of the latter detector averages over time the intensity of a corresponding part of the received optical signal to provide a measure of the energy and, hence, the phase-shift of the signal part. The measured phase-shift then serves as an indication of the range of the corresponding surface point.
The '616 patent mentions that in order to provide more reliable and broader range detection, multiple measurements can be made, one measurement using a cosine modulation function and the other a sine modulation function at the receiver. This increases the so-called "ambiguity range", i.e., the distance before which an ambiguity arises because of a repeat in the modulation values, to a full 360.degree. in phase angle corresponding to a distance of one-half of the modulation frequency. It also enhances measurements where the cosine function changes slowly, and, therefore, loses accuracy.
Additionally, the '616 patent also mentions that greater immunity to noise can be realized by making measurements at two frequencies which are different integer multiples of the same common frequency. In this case, processing can be carried out such that a long ambiguity interval characteristic of the lower common frequency is realized, while good range resolution characteristic of the higher integer multiple frequency is also realized.
While the system of the '616 patent can provide range resolution equal to that of the above-discussed digital system, the need to provide continuous optical laser illumination has undesirable consequences. First, it makes the system sensitive to relative motion between the system components and the object surface. This motion sensitivity can lead to erroneous and less reliable range determinations. Also, the laser power used must be relatively low in order to preserve the integrity of the object surface during continuous illumination. This makes the system more susceptible to background illumination and other noise sources. Additionally, the system is not readily adaptable to multiple channel use.
It is, therefore, an object of the present invention to provide an improved apparatus and method for use in determining the spatial location of the points on an object surface.
It is a further object of the present invention to provide an improved apparatus and method as mentioned in the previous objective and which are less susceptible to relative motion between the apparatus and object surface point and to background noise and which provide high range resolution.